The present invention relates to a demodulator for use in a multiple channel frequency modulation stereo receiver. More particularly, it pertains to a doubly balanced demodulator for developing a plurality of audio difference signals for processing by such a receiver.
Present-day broadcast FM stereo features the transmission of a two-channel coherent stereo signal, the modulation function of which may be represented as follows: EQU M'(t) = K'(L+R) + K"(L-R) sin.omega..sub.s t, (1)
where L represents a left side audio signal, R represents a right side audio signal, .omega..sub.s is the radian frequency of a suppressed carrier amplitude modulated subcarrier signal, t is time, and K' and K" are constants. A two-channel stereo receiver responds to a stereo broadcast by demodulating the sum and difference audio terms and then matrixes those two terms in order to yield the fundamental left and right audio signals L and R. This same receiver responds to a monaural FM broadcast by reproducing the same monaural audio signal in both of its output channels.
On the other hand, a monaural FM receiver responds to a two-channel broadcast stereo signal by deriving only the sum term (L+R) represented in equation (1) to reproduce an audio signal representative of the monaural program. The two-channel signal thus is fully compatible with the monaural signal so that a receiver properly designed for one will also receive the other. Further detailed discussion of the foregoing two-channel transmission system and exemplary disclosures of transmitters and receivers for use therewith will be found in the following U.S. Pat. Nos.: 3,257,511 -- Adler et al.; 3,257,512 -- Eilers; 3,129,288 -- DeVries; and 3,151,218 -- Dias et al., all of which patents are assigned to the same assignee as the present application.
In the last few years, interest has been evidenced in recording systems wherein a four-channel stereo signal is recorded on magnetic tape. The four different audio signals represent sources respectively located at the left front, right front, left rear and right rear of an originating point. By using four similarly located pick-ups to effect a four-channel recording and, subsequently, a playback arrangement having four separate loudspeakers similarly distributed around a listening point, a four-channel reproduction is obtained.
The advent of four-channel stereo recording and reproduction has naturally led to consideration of the desirability of transmitting and receiving four-channel stereo signals by radio. Because two-channel stereo is now being broadcast by many FM transmitting stations, attention has been directed particularly to the possibility of utilizing broadcast stations in that category of service for the transmission of four-channel stereo in addition to, or instead of, the transmission of two-channel stereo or monaural signals. To accomplish such a transmission a complex composite baseband signal must be developed in order to accommodate the additional signal components necessary to convey four separate channels of information. At the same time, it is desirable that any four-channel approach be fully compatible both with present two-channel stereo and monaural, so that receiver obsolescence is avoided.
It is also desirable from the standpoint of broadcast station economics, that a commercial four-channel stereo system include provision for an SCA (Subsidiary Communications Authorization) channel. The above-mentioned U.S. Pat. No. 3,944,747 of Eilers, as well as U.S. Pat. No. 3,902,018 of Fockens, both describe four-channel compatible FM stereo systems capable of accommodating SCA.